Effect of Alkaline Extrusion Temperature on Rheological Properties of Andean Corn Dough ^†

Abstract

The application of alkaline extrusion in whole corn flour not only produces partial gelatinization of starch but also favors interactions between its components and releases natural hydrocolloids, modifying the rheological properties and suitability for application in gluten-free pastas or bakery products. The intensity of these modifications and therefore their rheological quality depend on the extrusion conditions. This work aimed to study the effect of alkaline extrusion temperature (70, 80 and 90 °C) at 40% feed humidity on the rheological properties of Cuzco corn flour and its dough. The increase in extrusion temperature had a significant effect (p < 0.05) on the degree of gelatinization of the flours (increase from 31.74 to 71.64%), which impacted their viscous properties. The degree of gelatinization, the formation of amylose–lipid–protein complexes and the soluble fiber content modified the rheological properties of the dough, decreasing the elastic modulus with increasing extrusion temperature. The most cohesive and elastic doughs were obtained at a lower temperature (70 °C), which presented greater resistance to kneading. This study will expand the use of whole Andean corn flour in gluten-free dough to obtain pastas and/or bakery products, reducing the use of food additivess.

1. Introduction

Nixtamalization is a widely used technology in Central America in the processing of corn to produce “tortillas” [1]. Alkaline extrusion has emerged as a sustainable alternative to traditional nixtamalization, as an ecological hydrothermal treatment that allows for the utilization of whole grains [2]. The combination of mechanical stress with heat treatment and an alkaline agent produces changes and promotes the interaction between flour components, modifying its functional properties to be incorporated in gluten-free foods [3]. The presence of the alkaline agent solubilizes components of the pericarp, generating natural hydrocolloids that influence the rheological properties of the doughs [4]. The degree of starch gelatinization and the amylose–lipid and calcium–starch interactions are susceptible to small changes in processing conditions (moisture content and temperature); therefore, maintaining optimal extrusion conditions is important [3].

The study of the rheological properties of the dough allows for the determination of its functional characteristics. Viscoelastic parameters such as the elastic (G′) and viscous (G″) modulus are fundamental in the determination of the quality of corn doughs [3] and depend on the intensity of mechanical stress that affects the cohesive and elastic properties of the doughs [2,4,5].

Hydrothermal treatments applied to gluten-free flours, such as corn flour, are essential to adjust their functional properties according to culinary and processing needs. Although these treatments have been investigated to improve the technological aptitude of gluten-free flours to develop many products [1,2,4,5], there are few studies that apply alkaline extrusion to produce fresh, laminated and easy-to-cut doughs. Therefore, the present work aimed to study the effect of alkaline extrusion temperature on the rheological properties of Andean corn flour, Cuzco race and its doughs.

2. Materials and Methods

2.1. Raw Materials

White Andean corn of the Cuzco variety was used, supplied by the CAUQUEVA cooperative (Maimará, Jujuy Province, Argentina) and the INTA-IPAF NOA (Maimará). Whole-grain milling took place in a hammer mill (Polymix PX-MFC-90 D Kinematica) until a flour with a particle size ≤ 450 µm was obtained, which was sieved through mesh No. 40 (ASTM-E-11-61).

2.2. Alkaline Extrusion of Corn Flours

A total of 12 h before the process, 0.25 g of Ca (OH)₂/100 g of flour was added to each whole-meal flour sample, and then they were conditioned at 40% moisture. Each sample was mixed for 3 min and stored in a polyethylene bag in the refrigerator at 5 °C. To obtain alkaline-extruded corn flour (HMEA), a Brabender extruder (KE 19/25D, Germany) with a single screw of a 2:1 nominal compression ratio was used. The feed and extrusion rates were set at 20 rpm and 60 rpm, respectively. The extrusion was conducted at 70, 80 and 90 °C. The extrudates were formed through a nozzle with a diameter of 3 mm and were collected on a tray for subsequent drying.

2.3. Composition of Macronutrients in Processed Flours

For the determination of macronutrients, the AOAC [6] analytical methods were employed: proteins (AOAC International, 2005b) and lipids (AOAC International, 2005c). The total dietary fiber (TDF) and insoluble dietary fiber (IDF) contents were determined according to AACC 32-05 (2000), using the enzymatic–gravimetric method. The soluble dietary fiber (SDF) content was obtained by the difference between the TDF and IDF. The experiment was carried out in triplicate.

2.4. Physicochemical Properties of Processed Flours

2.4.1. Degree of Gelatinization (DG)

The colorimetric method developed by Birch and Priestley (1973) was used, based on the formation of an amylose–iodine complex.

2.4.2. Viscous Properties of Processed Flours

The viscous properties were determined using a Rapid Visco Analyser (RVA) (RVA series 4500, Perten instruments) following the methodology of Method 76.21.01, AACC, 2000 [7].

2.4.3. Subjective Water Absorption Water (SWAC)

SWAC was determinated according to Gaitan-Martinez et al. [8].

2.5. Rheological Properties of Doughs

The loss modulus (G″) and storage modulus (G′) of the rehydrated masses were determined using the SWAC methodology. The moduli were measured using a rheometer (TA Instrumen 5 AR 1000), in accordance with the methodology reported by Platt-Lucero et al. [4].

Textural Properties

A Texture Profile Analysis (TPA) was conducted on dough discs (3 cm in diameter and 1 cm in height) which were subjected to a double-compression cycle using 40% deformation of the original height with an SMSP/50 probe and a 25 kg load cell. The test speed was 0.5 mm/s. Six discs were tested for each formulation, from which the parameters of elasticity and cohesiveness were obtained.

2.6. Statistical Analysis

The data obtained were statistically treated by analysis of variance, while the means were compared by using the LSD Fisher’s test at a significance level of 0.05 using the statistical software INFOSTAT—Version 2017p (Facultad de Ciencias Agropecuarias, UNC, Cordoba Argentina). All experiments were performed in triplicate, and mean values ± standard deviation were reported.

3. Results and Discussion

3.1. Macronutrient Composition of Processed Flours

Table 1. Macronutrient composition and degree of gelatinization of processed flours.

Muestras	Free Lipids (%)	Protein (%)	TDF (%)	IDF (%)	SDF (%)	DG (%)
HCEA 70-40	2.39 ± 0.14 b	11.11 ± 0.14 ab	11.23 ± 0.01 ab	6.12 ± 0.03 a	5.11 ± 0.01 d	31.74 ± 0.64 a
HCEA 80-40	2.14 ± 0.16 a	11.27 ± 0.14 bc	11.01 ± 0.50 a	7.70 ± 0.14 b	3.31 ± 0.36 c	36.47 ± 0.03 b
HCEA 90-40	2.14 ± 0.35 ab	11.10 ± 0.04 ab	11.85 ± 0.07 c	9.82 ± 0.02 c	2.03 ± 0.05 b	71.64 ± 0.03 c
HC nativa	4.08 ± 0.11 c	11.09 ± 0.19 ab	11.55 ± 0.25 b	10.5 ± 0.10 d	1.05 ± 0.14 a	ND

Different letters in the same column indicate significant differences. (p < 0.05). FDT: total fiber dietary; FDI: insoluble fiber dietary; SDF: soluble fiber dietary; DG: degree of gelatinization; ND: no determinate.

presents the macronutrient content in native and treated Cuzco corn flours. The free lipid content was found to be between 2.14 and 2.39% in all treated flours, significantly lower than the lipid content in the native flour (4.08%). No significant differences were observed between processing temperatures. The decrease in lipids during extrusion can be attributed to lipid saponification with the alkaline agent and the complexes’ (amylose–lipid and ternary complexes such as starch–protein–lipid) formation from gelatinization [7]. These complexes could participate in the formation of structures that improve the extensible and cohesive properties of gluten-free doughs.

The total dietary fiber (TDF) of the processed flours did not present significant differences (p < 0.05) compared to the native flour. However, an increase in soluble dietary fiber (SDF) and a decrease in insoluble dietary fiber (IDF) were observed, suggesting a conversion between them due to the alkaline agent that induces the hydrolysis of hemicellulose and forms soluble gums that improve the texture of the doughs [2]. In addition, the SDF content increased significantly (p < 0.05) with decreasing extrusion temperature, evidencing a range from 1.05 to 5.11, which is consistent with that reported by Tabligbohmany et al. [8], who indicated that fiber solubilization is more linked to mechanical stress than to thermal energy because the greater frictional force causes the breakdown of the chemical bonds of the macromolecules of insoluble fiber.

3.2. Physicochemical Properties of Alkaline-Extruded Flours

3.2.1. Degree of Gelatinization

The degree of gelatinization (DG) (Table 1) increased significantly (p < 0.05) with the processing temperature and its values ranged from 31.74 to 71.64%. Topete-Betancour et al. [9] mentioned a value of approximately 30% as adequate to obtain easy-to-laminate doughs.

3.2.2. Viscous Properties

Significant differences (p < 0.05) were observed in the parameters of the profile between processed and native flours (Figure 1); the last one presented higher values of VP, BD, SB, MV and FV, but lower PT values. This could be due to the fact that most of its starch granules were intact. It is observed that VP decreases (p < 0.05) with extrusion temperature. The lower processing temperature provides flours with a lower degree of gelatinization; therefore, they have more starch available to develop higher viscosities. The FVs were also influenced by the extrusion temperature, observing that flour HCEA 70-40 developed higher FVs than HCEA 90-40. This behavior would indicate that flours with a higher degree of gelatinization would produce very soft doughs that could fall apart.

3.3. Rheological Properties

In all samples, G′ > G″ was observed throughout the frequency range studied (Figure 2). This reveals that the system has a viscoelastic solid behavior, with strains that are essentially elastic and recoverable [9]. Although the dough is gluten-free, this behavior demonstrates the presence of a network with interactions that stabilize the system at the stresses utilized. The alkaline treatment causes ionization of some hydroxyl groups in the starch, allowing for the formation of Ca–starch or Ca–protein cross-linking, resulting in a stronger gel network with a higher G′ and G″ modulus. These results are in agreement with those reported by Rolandelli et al. [10].

The variation in extrusion temperature changes the values of the modulus, but not the general shape. HCEA 70-40 had the highest elastic modulus possibly due to the higher soluble dietary fiber (SDF) content, improving the elasticity of the dough.

Textural Properties

Table 2. Subjective water absorption capacity and textural properties of processed flour doughs.

Muestras	SWAC (%)	Elasticity (mm)	Cohesive
HCEA 70-40	0.79 ± 0.00 b	0.29 ± 0.00 b	0.33 ± 0.02 b
HCEA 80-40	0.80 ± 0.00 c	0.29 ± 0.02 b	0.38 ± 0.04 c
HCEA 90-40	0.67 ± 0.01a	0.26 ± 0.01 a	0.28 ± 0.02 a

Different letters in the same column indicate significant differences (p < 0.05).

shows that SWAC values decreased with the increasing processing temperature. This behavior may be responsible for the lower elasticity and cohesiveness observed in dough elaborated with flour extruded under 90-40 conditions. The 70-40 and 80-40 flours did not present significant differences in terms of elasticity, but the second flour presented higher cohesiveness. This could be related to the higher soluble fiber content, which provides greater elasticity. The results indicate that HCEA 70-40 and HCEA 80-40 flours form more cohesive doughs. The elasticity determined in HCEA 70-40 and 80-40 (0.29 mm) was within the ranges obtained by Topete-Betancourt et al. [9].

4. Conclusions

The extrusion conditions of 70 and 80 °C with 40% moisture provided flours with properties suitable for the formation of laminate and resistant doughs. These conditions favored the hydrolysis of pericarp components, generating compounds that act as hydrocolloids that interact with water, contributing to a more cohesive and elastic dough. These findings underline the relevance of carefully determining the effects of processing on the physicochemical properties of flours to ensure the quality of the final products.